1. Field of Invention
The invention relates to transistor base ballast circuits.
2. Related Art
Heterojunction bipolar transistors (HBTs) are often formed as a group of associated transistor cells. Thermal runaway is a well-known problem in HBT amplifiers using two or more transistor cells connected in parallel. Slight physical variations in the transistor cells may cause one particular transistor cell to conduct more current than the other cells. Energy from the increased current is dissipated as heat. As the cell temperature increases, the cell""s collector current also increases. In a runaway situation, therefore, a particular transistor cell eventually conducts more current than it is capable of dissipating and the transistor cell destroys itself.
There are several known methods to prevent such thermal runaway. One method is to control the base-emitter voltage by connecting each transistor cell emitter to ground via a resistor. The transistor base terminals are biased at a constant DC voltage. As collector current increases due to heating, the voltage drop across the emitter resistor increases. This increased voltage drop reduces the base-emitter voltage and consequently reduces the transistor""s collector current. Thus heating is slowed. Among the disadvantages of such emitter ballast resistors are reduced radio frequency (RF) gain, increased thermal resistance of the transistor-emitter resistor combination, and difficulty in fabricating emitter ballast resistors with the required low resistance (e.g., about 2 ohms).
Another method used to prevent thermal runaway is the use of a base ballast resistor. As shown in FIG. 1, for example, ballast resistors 10,12 are coupled to the respective base terminals of HBT transistors 14,16. Capacitor 18 is coupled in parallel with resistor 10 and capacitor 20 is coupled in parallel with resistor 12. During operation, direct current from a bias voltage source (not shown) passes through resistors 10,12, thereby establishing a steady-state base-emitter voltage for each transistor 14,16. The RF input signal RFIN passes through capacitors 18,20. Transistors 14,16 amplify signal RFIN and consequently output signal RFOUT. If heating causes one of transistors 14,16 to conduct excessive collector current, a corresponding increase in base current results. This increased base current causes an increased voltage drop across the associated base resistor. The increased voltage drop lowers the base-emitter voltage, and thus reduces the collector current in the overheating transistor. Base ballast resistor circuits are further described in U.S. Pat. No. 5,321,279 and European Patent Application EP 0 736 908 A1, both of which are incorporated by reference.
The circuit shown in FIG. 1 has disadvantages. Capacitors 18,20 must be sufficiently large to provide an effective RF bypass. The use of capacitors 18,20 as reactive tuning elements (i.e., not a bypass element where capacitive reactance is near zero) to provide reactive matches is precluded due to the resistive losses in parallel. At frequencies in the low microwave range (e.g., 1-2 GHz) and below, the area required to form the capacitors on an integrated circuit becomes impractical. During 800 MHz operation, for example, if ballast resistors 10,12 are each 125 ohms then capacitors 18,20 should each be about 15-20 pF. Depending on the integrated circuit fabrication process used, the chip area required to provide such capacitors in a 16-cell transistor amplifier output stage could easily be 0.5 mm2.
FIG. 2 shows another base ballast resistor circuit configuration. In the circuit shown in FIG. 2, the nodes receiving the DC and RF input signals are separated. As shown in FIG. 2, the base terminal of each transistor 14,16 is connected to a unique resistor 22,24. The opposite terminals of resistors 22,24 are coupled together at a common DC bias input node. The base terminal of each transistor 14,16 is also coupled to one electrode of a unique capacitor 26,28. The opposite electrodes of capacitors 26,28 are coupled together at a common RF input node. Embodiments of the circuit shown in FIG. 2 are described in detail in U.S. Pat. Nos. 5,608,353 and 5,629,648, both of which are incorporated by reference. Capacitors 26,28 are smaller, and hence require less chip area, than capacitors 18,20 shown in FIG. 1 because capacitors 26,28 are typically reactive tuning elements.
Thus a need exists for a circuit providing effective base ballast capabilities and small size for RF signal frequencies at and below the low microwave range. Such a circuit should combine the benefits of the use of reactive tuning elements with a single input node receiving both the required DC bias signal and RF input signal for amplification.
In a circuit using two or more transistor amplifier cells, a base ballast resistor is coupled between the base terminal of each unique transistor cell and a single input node receiving both a DC bias signal and an RF signal. Each ballast resistor is bypassed with a series-coupled inductor and capacitor that provide a resonant, low-loss RF signal current path between the input node and the base terminals of the transistor cells. In one embodiment a unique inductor and capacitor pair are used to bypass each base ballast resistor. The use of an inductor allows the capacitor to be made smaller than if no inductor is used. In another embodiment, a common inductor and parallel capacitors are used to form the resonant current paths between the input node and the base terminals. The use of a common inductor further reduces the chip area required by the base ballast circuit since the common inductor can be made smaller than the separate inductors.